Automotive air conditioners have used chlorofluorocarbon gases, such as HFC-134a (R-134a), as refrigerants.
In order to improve ride comfort, automotive air-conditioning piping uses highly vibration-absorbing rubber hoses. Japanese Patent Publication 2007-15245A (Patent Literature 1) discloses a rubber hose that includes an innermost layer of a polyamide resin superior in terms of gas barrier property and vibration durability such as impulse resistance so as to prevent the refrigerant from leaking. The rubber hose further includes a rubber inner tube layer on the polyamide resin layer, a reinforcing yarn layer made of an organic fiber, such as PET, on the rubber inner tube layer, and a weather-resistant EPDM rubber layer on the reinforcing yarn layer.
Japanese Patent Publication 2000-120944A (Patent Literature 2) discloses a refrigerant-permeation resistant flexible refrigerant transporting hose that includes an innermost gas barrier layer made of a polyamide resin containing a polyolefin-based elastomer as a flexibility-imparting agent.
A polyamide resin may be degraded by a chlorofluorocarbon used as a refrigerant or an oil from a compressor. Even if a polyolefin-based elastomer is added to the polyamide resin, such degradation may not fully be prevented.
Known refrigerant transporting hoses do not, therefore, exhibit sufficient durability in some cases.
For example, if a trace amount of an acid component is present in an air conditioning system, the acid component may seriously degrade the polyamide resin composition to the extent that the hose cannot be used. The acid component can be an extreme pressure agent contained in a compressor oil enclosed together with a refrigerant. Therefore, some of the known refrigerant transporting hoses cannot exhibit sufficient durability in practice and may become unusable, depending on environmental conditions and the type of oil used in the air conditioning system.
Patent Literature 3 (Japanese Patent Publication 2010-249316A) discloses a refrigerant transporting hose whose durability has been enhanced by preventing the gas barrier layer made of a polyamide resin composition from being degraded by a refrigerant or a compressor oil. In the refrigerant transporting hose of Patent Literature 3, the polyamide resin composition for forming the gas barrier layer contains at least one metal compound selected from the group consisting of hydroxides, oxides and carbonates of divalent or trivalent metals in a predetermined proportion.
Since the polyamide resin composition of Patent Literature 3 contains such a metal compound, deterioration of the gas barrier layer made of the polyamide resin composition by a refrigerant or a compressor oil is suppressed or prevented (suppress or prevent is hereinafter simply expressed as “prevent”) effectively, and the durability of the refrigerant transporting hose is enhanced.
It is not clear in detail how the above-mentioned metal compounds prevent the deterioration of the polyamide resin by a refrigerant or a compressor oil. It is, however, supposed that the metal compound in the polyamide resin composition acts as an acid acceptor, a halogen acceptor or the like to trap acid components, halogen components and other degrading components contained in the refrigerant or the oil and is thus effective in preventing the deterioration of the polyamide resin.
The gas barrier layer of a refrigerant transporting hose, made of the gas barrier layer-forming polyamide resin composition of Patent Literature 3 containing one or more of the above-mentioned metal compounds is prevented from being deteriorated by a refrigerant or a compressor oil. However, the above-mentioned metal compounds are difficult to disperse uniformly in a polyamide resin. In a polyamide resin composition containing a relatively large amount of a metal compound to enhance durability, the dispersibility of the metal compound is not good in the polyamide resin composition, and a nonuniform phase portion of the metal compound may be formed in the polyamide resin composition. Accordingly, in a refrigerant transporting hose including a gas barrier layer made of such a composition, breakage can occur from such a nonuniform phase portion. Thus, the resin can be broken by a fatigue test such as repetitive pressurizing test (impulse test) and may not exhibit satisfactory performance. More specifically, if the metal compound is not sufficiently dispersed in the polyamide resin composition, the state of the surface portion (surface condition) of the gas barrier layer, which is a thin film formed by extruding the polyamide resin composition, becomes worse and the impulse resistance of the refrigerant transporting hose may be degraded.
Patent Literature 4 (Japanese Patent Publication 2011-6680A) discloses a method for producing a polyamide resin composition, in which the dispersibility of the metal compound in the polyamide resin composition of Patent Literature 3 is enhanced. The method of Patent Literature 4 produces a polyamide resin composition containing a polyamide resin, a polyolefin-based elastomer, at least one metal compound selected from the group consisting of hydroxides, oxides and carbonates of divalent or trivalent metals. In this method, the metal compound and the polyolefin-based elastomer are mixed and kneaded, and the resulting mixture is mixed and kneaded with the polyamide resin.
As disclosed in Patent Literature 4, in the method for producing the polyamide resin composition containing a polyamide resin, a polyolefin-based elastomer, and any of the above-mentioned metal compounds, the metal compound is uniformly dispersed in the resin composition by first mixing and kneading the polyolefin-based elastomer and the metal compound, and then mixing and kneading the polyamide resin with the resulting mixture. Thus, the impulse resistance of the refrigerant transporting hose is enhanced.
The above-mentioned metal compounds are easy to disperse uniformly in a polyolefin-based elastomer. Accordingly, as disclosed in Patent Literature 4, the metal compound is uniformly dispersed in the resin composition by first mixing and kneading the metal compound with the polyolefin-based elastomer. Consequently, the occurrence of a defect portion that exhibits a poor surface condition and can cause breakage is prevented.
The deterioration of the polyamide resin composition of a refrigerant transporting hose results mainly from degradation of the polyamide resin by acid components. The acid component that will degrade the polyamide resin composition may penetrate into the polyolefin-based elastomer phase as well as the polyamide resin phase, and, in some cases, may reach the polyamide resin phase after penetrating the polyolefin-based elastomer phase. The metal compound dispersed in the polyolefin-based elastomer phase can trap the acidic component penetrating the polyolefin-based elastomer phase, thus preventing the deterioration of the polyamide resin effectively.
In the polyamide resin composition of Patent Literature 4, the metal compound is dispersed mainly in the polyolefin-based elastomer phase. In the gas barrier layer made of the polyamide resin composition, the metal compound traps the acid component penetrating the polyolefin-based elastomer phase, thus preventing the deterioration of the polyamide resin.
In order that a polyamide resin composition for the gas barrier layer of a refrigerant transporting hose ensures such a high heat resistance and durability that the polyamide resin composition can withstand high-temperature and high pressure conditions, an anti-aging agent such as copper iodide or potassium iodide may be added to the polyamide resin composition (Patent Literature 5).
The present inventors, however, have found that a polyamide resin composition prepared by further adding an ant-aging agent to a polyamide resin composition containing any of the metal compounds disclosed in Patent Literature 3 has to be improved more than the case where an ant-aging agent is added to a polyamide resin composition not containing a metal compound.
Thus, there has not been provided a polyamide resin composition exhibiting both a heat resistance sufficient to withstand high temperature and high pressure conditions and a durability against deterioration by acid components or moisture derived from refrigerants or compressor oils.